Modal actuators employing a plate-like piezoelectric member that resonates in bending are discussed in WO01/54450. FIG. 1a shows a known modal moving coil actuator 10 which is symmetrical about its central axis and which is mounted to a circular panel diaphragm 12 by a coupler 14. The actuator comprises a magnet assembly and a voice coil assembly and is similar to the actuator described in pending UK application 0617551.7.
The magnet assembly comprises four pairs of coaxially mounted ring magnets 16,18,20,22 and a pair of low carbon steel cup plates 24,26. Each pair of magnets comprises an upper ring magnet mounted to the upper cup plate 24 and a lower ring magnet mounted to the lower cup plate 26. The upper and lower ring magnets in each pair are separated by an annular planar air gap into which the coil assembly is mounted.
The coil assembly comprises three voice coils 30, 32, 34 printed on a self-supporting annular planar substrate 28. The voice coils each apply a force (A, B or C respectively) to the substrate. Along its inner edge, the substrate 28 is clamped between the upper and lower cup plates 24,26. Since the actuator is inertial, an opposing force equal in magnitude to the sum of the three forces A, B and C, must be applied to the magnet assembly at this inner edge. At the outer edge, the substrate is attached to the coupler 14 and couples the actuator to the diaphragm whereby the power from the actuator is delivered to the diaphragm.
Since the substrate is not perfectly rigid, such modal actuators exhibit secondary or anti-resonances where almost no force is transmitted to a load. Accordingly, the force provided by such actuators varies with frequency to have nulls of force. For example, FIG. 1b shows the variation in force with frequency available at the coupler in FIG. 1a. The response is far from satisfactory due to a characteristic dip present at 6.5 kHz which is caused by an anti-resonance of the substrate. There are also peaks in the response but it is known that these may be minimised by adding damping.
It is an object of this invention to improve the transfer of force for any actuator, particularly modal transducers.